1. Field of the Invention
The present invention relates to an exposure apparatus adapted for use in photolithographic manufacture of semiconductor devices or liquid crystal display devices, and more particularly to a scanning exposure apparatus of so-called slit scan method or step-and-scan method in which a mask and a photosensitive substrate are scanned in synchronization with respect to a rectangular or arc-shaped illumination area, whereby patterns on the mask are exposed in succession onto said substrate.
2. Related Background Art
The exposure apparatus conventionally used in the manufacture of the semiconductor devices or the like is principally a projection exposure apparatus of collective exposure method (called stepper) in which the patterns of a reticle (or a photomask) are exposed, by the step-and-repeat method, through a projection optical system to each shot area on a wafer (or a glass plate) coated with photoresist. In such projection exposure apparatus, there is provided an illumination intensity control mechanism for maintaining the exposure amount to each shot area on the wafer within an appropriate range.
FIG. 12 shows a stepper or a projection exposure apparatus provided with a conventional illumination intensity control mechanism. Illuminating light from a mercury lamp 1 is condensed by an elliptical mirror 2, then transmitted by a condensing optical system 3a and a condensing filter system 3 consisting of an optical filter 3b for selecting a desired wavelength region (for example i-line) and reaches a shutter 4, which is opened and closed by a shutter control mechanism 5 based on instructions from a timer control system 6. When the shutter 4 is open, the illuminating light is converted into a substantially parallel light by an input lens 7 and enters a fly's eye lens 8.
On the exit plane of the fly's eye lens 8 there are formed a plurality of images of the light source, whereby the illumination intensity distribution of the illuminating light is made uniform on a reticle 19. Close to said exit plane there is provided a turret plate 22 having plural illuminating diaphragm apertures, and the illuminating light passing one of said illuminating diaphragm apertures enters a mirror 9 of a reflectance of ca. 98%. The illuminating light reflected by the mirror 9 is transmitted by a first relay lens 13 and is limited to a predetermined illumination area on an illuminating blind (variable field diaphragm) 14 controlled by a blind drive system 15, and the illuminating light passing said illuminating blind 14 is relayed by a second relay lens 16, a mirror 17 and a condenser lens 18 and illuminates the illumination area on the reticle 19 with a uniform illumination intensity distribution. Based on said illuminating light, the pattern of the reticle 19 is projected, through a projection optical system 20, with a size reduction for example to 1/5, onto each shot area on a wafer 21.
Leaking light transmitted by the mirror 9 of the reflectance of ca. 98% is transmitted by a condenser lens 10 and enters an integrating sensor 11 consisting of a photoelectric detector, of which output signal is supplied to an illumination intensity calculating system 12. The integrating sensor 11 is so positioned as to be conjugate with the pattern bearing face of the reticle 19, and there is memorized, in advance, a conversion coefficient between the exposure energy at the wafer 21 and the illumination intensity on the integrating sensor 11. Thus the exposure energy per unit time on the wafer 21 can be obtained by multiplying said conversion coefficient in the illumination intensity calculating system 12. The information of said exposure energy per unit time is supplied to a main control system 25, which provides the timer control system 6 with an exposure time obtained by dividing the appropriate exposure amount on the wafer 21 with said exposure energy per unit time. In response the timer control system 6 opens the shutter 4 for said exposure time, whereby the integrated exposure amount on the wafer 21 is controlled to an appropriate amount.
Also for improving the resolution and the depth of focus for periodic pattern of a small pitch, there have recently been proposed a modified light source method of constructing the illuminating diaphragm with plural apertures deviated from the optical axis (cf. Japanese Patent Application Laid-open No. 4-225358) and an annular illuminating method of employing an annular illuminating diaphragm aperture. Therefore, in the configuration shown in FIG. 12, the turret plate 22 includes apertures for such modified light source method and for the annular illumination method. The main control system 25 sends the information on the pattern to be exposed to an illuminating diaphragm aperture control system 24, which in response rotates the turret plate 22 by a motor 23, thereby setting a required diaphragm aperture at the exit plane of the fly's eye lens 8.
However, such change in the shape of the illuminating diaphragm aperture results in a variation in the illumination intensity on the wafer 21, because of the change in the number and distribution of the light source images in the illuminating diaphragm aperture. In spite of such variation in the illumination intensity, an appropriate exposure amount is obtained in each shot area on the wafer, by monitoring the output signal of the integrating sensor 11, having the light-receiving face conjugate with the exposed surface of the wafer 21, and regulating the open time of the shutter 4 according to said output signal.
In such collective exposure method, the unevenness in the illumination intensity in the irradiated plane is suppressed for example by an optical integrator (fly's eye lens 8). Also as the shutter 4 is controlled indirectly by monitoring the integrated exposure amount on the wafer 21, the stability of the illumination intensity in time in each shot area on the wafer 21 has not been a problem. Furthermore, even if the illuminating condition is varied, for example by the modified light source method, between the integrating sensor 11 and the illuminating light source (mercury lamp 1), there merely appears a variation in the time required for the integrated exposure amount to reach the target value, and there has not been encountered any drawback.
Such conventional technology is based on the use of an illumination intensity control mechanism in an exposure apparatus of the collective exposure system in which the exposure of each shot area on the wafer is achieved by a single open/closing operation of the shutter while the reticle and the wafer remain still with respect to the projection optical system. Such collective exposure system has not been associated with any particular drawback as regards the illumination intensity control.
On the other hand, each chip pattern of the semiconductor devices has recently become larger, and the projection exposure apparatus is required to efficiently project the pattern of a larger area onto the wafer. For attaining such larger area, it is particularly necessary to maintain the distortion within a predetermined amount over the entire area. For reducing the distortion and expanding the projection area, attention is being attracted to the scanning exposure apparatus of so-called step-and-scan system or slit scan system, in which, after each shot area on the wafer is stepped to a scan start position, the reticle and the photosensitive substrate are moved in a synchronized scanning motion with respect to a rectangular, arc-shaped or plural-trapezoidal illumination area (hereinafter called "slit-shaped illumination area") whereby the pattern of the reticle is transferred in succession to the shot areas.
The conventional technology, if applied to such scanning exposure apparatus, will result in various drawbacks. Firstly, in the scanning exposure, as each shot area of the wafer is moved in a scanning motion with respect to a slit-shaped exposure field shorter than the length of said shot area, the control of the integrated exposure amount in each shot area is achieved by maintaining the integrated exposure amount, within said slit-shaped exposure field, constant on all the points on the wafer. If the integrated exposure amount varies from point to point on the wafer, the integrated exposure amount will become uneven within each shot area, resulting in an error similar to the unevenness in the illumination intensity in the irradiated plane in the exposure apparatus of the collective exposure system.
In the collective exposure system, the control of the integrated exposure amount is achieved by the shutter 4, but, in the scanning exposure system said control is achieved by moving the reticle and the wafer in a scanning motion with a predetermined constant speed, instead of the open/closing operation of the shutter. For this reason, it is difficult to attain fine adjustment of the integrated exposure amount by time. Consequently in the scanning exposure system, it is necessary to control the illumination intensity so as to continuously maintain the stability of the illumination intensity in time, during the exposure of each shot area. In the collective exposure system, there is already known a constant illumination intensity control method for constantly monitoring the illumination intensity of the illuminating light and effecting feedback of the obtained result to the power source of the illuminating light source thereby controlling the electric power supplied from said power source to the illuminating light source, but such method is not applicable directly to the scanning exposure apparatus.